Obtain external IP address

  http://ifconfig.me
  http://checkmyip.com
  http://checkip.dyndns.org|sed -e 's/.*: //;s/<.*//'
  http://api.ipify.org

Simple server which listens on a port and prints out received data

  nc -l -p portnumber

Show apps that use internet connection at the moment.

  lsof -P -i -n | cut -f 1 -d " "| uniq | tail -n +2

Block known dirty hosts from reaching your machine

  wget -qO - http://infiltrated.net/blacklisted|awk '!/#|[a-z]/&&/./{print "iptables -A INPUT -s "$1" -j DROP"}'

smtp server

  python -m smtpd -n -c DebuggingServer localhost:1025